Further advance/mortgage for building extension

biscuitmonster_2

We are looking to build onto our existing home, and we are looking at around £80-90k to finish it.

The property is currently valued at approx £190k, and we've got £88k to pay. I've got 2 mortgages running on this, one is running 2% above the base rate (Approx £58k) I can switch any time but offers borrowback on an overpayment which we have (£19k) and I can pay in as much as I like. The other one is at 2.6% for the remaining £30k which expires June 2020.

I'm looking to take about an extra £60000 from the mortgage to start the build and probably use the overpayment reserve to finish it. I'll then have to keep paying out until its finished.

Affordability shouldn't be a problem but I'm just considering whether its just better to go with the current lender on both products (nationwide) or whether I need to see a mortgage advisor to see whether I would be better placed to go to another lender? I quite like the flexibility of the first mortgage product with the overpayment reserve, but realise that once I've got this built and revalued, then I should be in a decent LTV position again, and can maybe arrange a better longer term rate for all products.

All information most welcome. Thanks BM.